BSENISO5983-2:2009 ISO5983-2:2009(E) 02:01 /2009 3 Terms and definitions For the purposes of this document, the following terms and definitions apply /08/ 3.1 04/ nitrogen content mass fraction of nitrogen determined by the procedure specified in this part of iso 5983 NOTE The nitrogen content is expressed as a percentage mass fraction or in grams per kilogram as 3.2 crude protein content NOTE The crudeproteincontentis expressed as apercentagemassfractionoringramsperkilogram rsion Principle The test portion is digested using a block digestion or equivalent apparatus. Concentrated sulfuric acid is used A copper catalyst is used to enhance the reaction rate. An excess of sodium hydroxide is added to the cooled digest to liberate ammonia. The liberated ammonia is distilled, using a manual, semi-automatic or fully automatic steam distillation unit. In the case of manual or semi-automatic steam distillation, distillation of the ammonia into an excess of boric acid solution is followed by titration with hydrochloric acid solution to a colorimetric endpoint. Where a fully automaticsystemisemployed,automatictitrationoftheammoniais carried out simultaneouslywiththe distillation and the endpoint of the titration can also be detected by means of a potentiometric pH system. The nitrogen content is calculated from the amount of ammonia produced. The crude protein content is obtained by multiplying the result by the conventional conversion factor of 6,25. NOTE In principle, sulfuric acid can also be used for the titration. 5 Reagents During the analysis, unless otherwise stated, use only reagents of recognized analytical grade and distilled or demineralized water or water of equivalent purity. 5.1 Kjeldahl catalyst tablets, comprising 3,5 g of potassium sulfate and 0,4 g of copper(ll) sulfate pentahydrate per tablet. These tablets are commercially available. Othertypes of tablet may be used provided that: Vocational a) sulfate pentahydrate can be dispensed using an integral number of whole tablets; and they do not contain salts of toxic metals such as selenium or mercury. 5.2 Sulfuric acid (H2SO4), at least 98 % mass fraction, nitrogen-free (P2o ~ 1,84 g/ml) Licensed copy: 5.3 2 @ ISO 2009 - All rights reserved t as of 04/08/2009 02:01, (c) BS EN ISO 5983-2:2009 ISO5983-2:2009(E) 5.4Antifoaming agent. A silicone preparation is recommended, e.g. with a mass fraction of 30 % aqueous emulsion. 5.5Sodium hydroxide (NaOH) solution, approximately 40 % mass fraction, nitrogen-free (<5μg of nitrogen per gram). 5.6 Indicator solutions. 5.6.1 Methyl red solution. Dissolve 100 mg of methyl red (C15H15N3O2) in 100 ml of ethanol or methanol. , Version correct 5.6.2 Bromocresol green solution. Dissolve 100 mg of bromocresol green (C21H14Br4OsS) in 100 ml of ethanol or methanol. 5.7 Concentrated boric acid solution, c(HgBO3) = 40,0 g/l. Dissolve 4o0 g of boric acid in about 5I to 6 I of hot water. Mix and add more hot water to a volume of about 9 1. Allow to cool to room temperature. Add 70 ml of the methyl red solution (5.6.1) and 100 ml of the bromocresol green solution (5.6.2) and mix. Dilute to a final volume of 10 I with water and mix well. Depending on the water used, the pH of the boric acid solution can differ from batch to batch. Often an adjustment with a small volume of alkali is necessary to obtain a positive blank (0,05 ml to 0,15 ml of titrant). The colour should Council, turn green when 100 ml of water are added to 25 ml of the boric acid solution. If still red, titrate with 0,1 mol/l NaOH until “neutral grey" and calculate the amount of alkali needed for the 1o I batch. of ammonia fumes during storage. 5.8Dilute boric acid solution, c(HgBO3) = 10,0 g/l (optional trapping solution for titrators that automatically S begin titration when distillation begins). B Dissolve 100 g of boric acid in about 5I to 6 I of hot water, mix and add more hot water to a volume of about , Vocational 9 1. Allow to cool to room temperature. Add 70 ml of the met